In order to improve wet properties, for example, water resistance of regenerated collagen fiber, it has been proposed to react the amino group or carboxyl group of collagen molecules with a methylol-containing compound as disclosed in JP-B-40-9062 (the term "JP-B" as used herein means an "examined published Japanese patent application") or to crosslink collagen molecules with formalin, a polyfunctional compound, or a basic chromate as disclosed JP-B-41-15259, JP-B-43-12633, and JP-B-47-14021. These proposals, as usually carried out in the leather industry, are effective on leather mainly comprising insoluble collagen, but not on regenerated collagen fiber comprising soluble collagen because the resulting fiber, when contacted with water, undergoes waving or shows high water absorption only to have insufficient water resistance, such as wet strength.